The CA protein is the major structural protein of the retrovirus core, yet its contribution to the virus replication cycle has never been determined. In the mature virion core, CA forms a protein shell that surrounds the ribonucleoprotein (RNP) complex of viral RNA and replication enzymes. The function of this capsid-like shell is entirely unknown. However, several lines of evidence indicate that the integrity of CA and the shell it forms is in some way necessary for reverse transcription of the viral genome. We have previously demonstrated that conservative amino acid substitutions in the major homology region (MHR, a sequence shared by most retroviruses, retrotransposons, and hepadnaviruses) cause a serious deficit in DNA synthesis ability of the particles and a failure of the virus to infect. Since these particles contain all the necessary protein constituents as well as an intact RNA genome, it appears that the MHR and CA itself has a critical role in forming the constituents of the core into an effective DNA synthesis machine. A sequence similarity with the P proteins of paramyxo- and rhabdoviruses suggests the possibility that the CA protein acts directly to facilitate utilization of the genomic RNA template by participating as a non-catalytic component of the polymerase complex. Our previous genetic studies of the MHR have yielded at least four revertant viruses that carry unmapped suppressors that fall outside of the gag gene, providing a compelling argument for functional interactions of CA with one or more other constituents of the virion. The major goals of the proposed research will be to determine the identity of the suppressors responsible, to test predictions that CA is in direct contact with the viral RNP, and to identify other portions of the CA carboxy-terminal domain that cooperate with the MHR to facilitate these activities. Achieving these goals will be a major step toward understanding one of the most mysterious aspects of the retroviral life cycle and the ultimate identification of new strategies for antiviral therapies.